Investment casting processes are extensively used in the production of nickel and cobalt base superalloy parts for gas turbine engines and other machines. In investment casting processes, a ceramic shell mold is formed around a wax pattern, with one or more ceramic cores precisely positioned within the pattern. After the mold-core composite is formed, the wax is removed from the composite by a firing operation. The core (or cores), in conjunction with the mold, define a cavity when the firing operation is complete. Molten metal is poured into and solidified in the cavity, and the cores are then removed from the metal casting by immersing the casting in an alkali leaching solution.
U.S. Pat. No. 4,836,268 to DeVendra describes porous casting cores which are said to be easier to leach from a metal casting than are dense cores. The DeVendra core has a closed cellular construction formed by a plurality of pores.
Some castings have very intricate internal passages produced by cores. In some part designs, these passages are configured such that the wall thickness between adjacent passages is as little as about 250 microns (10 mils). As is appreciated by those skilled in the art, the ability to produce such configurations requires a core which has excellent stability and strength, including the ability to withstand distortion during the firing and subsequent casting process.
In an effort to produce cores having the properties necessary to meet these needs, the casting industry has expended considerable effort to develop improved core compositions. One such composition is described in commonly assigned U.S. Pat. No. 4,989,664 to H. A. Roth. The Roth core composition includes a mixture of coarse and fine ceramic particles and high temperature stable fibers, the fibers present in an amount sufficient to preclude cracking and distortion of the core during the core firing process as well as during the metal casting process.
The leaching process for removing cores from metal castings is an important aspect of the overall process for making metal parts. If the core is not completely removed before the part, the residue which remains inside the casting can interfere with the proper performance of the part in service, which sometimes leads to premature failure of the part. To preclude such failures, castings are carefully inspected after leaching to make sure the core is completely removed. While the DeVendra core is said to have improved leachability characteristics as compared to fully dense cores, it is likely to have poor strength and stability owing to the abundance of pores in the core. It will therefore not likely be useful in making cores having a complex geometry. Accordingly, what is needed is a casting core having an optimum balance of strength and leachability.